Flotation is a widely utilised unit operation in mineral processing and is based upon the principle that different mineral species have different wetting characteristics. This difference in wetting characteristic can be used as a basis for separating the different mineral species of a milled ore because relatively unwetted or hydrophobic milled mineral particles adhere more strongly to a stream of gas bubbles, generally air, passing through a slurry of the milled mineral than those particles which are relatively wetted or hydrophilic.
The process is generally assisted by the addition of reagents, for example, depressants which reduce the flotation tendency of certain minerals such as pyrite and activators such as copper sulphate which activate, that is, assist minerals to float which do not have a tendency to do so even in the presence of collectors. Organic collectors such as sodium ethyl xanthate which enhance the tendency of mineral particles to adhere to bubbles of gas are also widely utilised.
The flotation operation is conducted in flotation cells and columns which contain a slurry of the milled ore to be separated into the constituent streams of concentrate and gangue. A gas, usually air, is sparged through the cell or column causing hydrophobic particles to selectively attach to air bubbles, generally with the aid of agents such as those described above. The hydrophobic particles collect in a froth layer at the top of the cell and are removed. The unfloated material is removed from the bottom of the cell from where it may be transferred to a further flotation stage in which the flotation conditions may be altered to selectively float the same or another desired mineral concentrate. Alternatively, the unfloated materials may be removed as a tails or gangue stream which may be used to fill desired mine shafts or for other forms of land reclamation.
A typical flotation process involves the separation of the constituents of a mixed ore such as an ore containing the minerals galena (lead sulphide), sphalerite (ZnS) and pyrite (FeS.sub.2). In a first stage, galena is floated by adding a xanthate collector (0.05-0.15 kg t.sup.-1 ore) to promote the flotation of galena. Sodium cyanide and zinc sulphate (0.05-0.15 kg t.sup.-1 ore and 0.5-1 kg t.sup.-1 ore respectively) are added to depress the pyrite and sphalerite. In a second stage, sphalerite is activated with copper sulfate to form a copper sulfide layer on the sphalerite grains which allows adsorption of the xanthate activator and flotation of a predominately zinc concentrate. Pyrite is recovered as a tailing.
Where the ore is more complex or the proportion of coarse particles is too high, regrinding and further flotation circuits may be required. Cleaner and scavenger flotation cells may also be required to maximise recovery of desirable mineral constituents. It is also to be noted that effective flotation requires careful control over chemical conditions such as pH which require an acid or lime to be added in conditioning stages prior to each flotation step.
In spite of the above precautions, the tails stream from a flotation circuit often contains appreciable amounts of valuable minerals and therefore, if the flotation operation is to be optimised in terms of economic efficiency, these minerals must be reclaimed to the maximum extent possible. Such an objective requires careful control over the flotation process both through judicious use of the above described agent, control over pH, Eh, and, consequently, the process chemistry. It will be appreciated, in this regard, that the above described agent are expensive and over use is to be discouraged.
A problem arises with certain minerals of economic importance, for example sphalerite (zinc sulphide), pyrite (iron (III) sulphide), arsenopyrite (iron arsenosulphide) and stibnite (Sb.sub.2 S.sub.3) in that such minerals have a poor tendency to float even in the presence of collectors. In these instances, it has been necessary to employ an activator such as copper sulphate to encourage flotation. The copper sulphate achieves this objective by encouraging the formation of a surface layer(s) of copper sulfide, a mineral which does have a tendency to float. In the case of sphalerite, the formation of this surface layer follows the chemical reaction. EQU ZnS+Cu.sup.2+ .fwdarw.CuS+Sn.sup.2+ (I)
Unfortunately, it has been found that copper sulphate must often be used in excess of the theoretical quantity required to enable the formation of sufficient coverage of the zinc sulfide with copper sulfide. As the operation is conducted at alkaline pH there is a tendency for hydroxylated copper species to form which may also react with other species such as cyanide and complex sulphated anions causing the activation process to become less efficient. Similar behaviour may be observed with other milled ones.